1. Field of the Invention
The present invention relates generally to a circuit device for driving a magnetometric sensor, and relates particularly to a circuit device for driving a SQUID (superconducting quantum interference device) which is used as a magnetic flux or field sensor.
2. Description of Related Art
A SQUID is a highly sensitive magnetometric sensor which can measure extremely small magnetic fields, and is usually provided with a magnetic flux locked loop, a so-called "FLL", in order to drive the SQUID acting as a magnetometric sensor. The driver circuit with the FLL uses a null balance method. In the driver circuit, a feedback signal obtained from a voltage signal detected by the SQUID is returned through an exciting coil to the SQUID and the operating point of the SQUID is automatically null-balanced to be locked at a trough or crest point of SQUID voltage (V) vs. magnetic flux (.PHI.) characteristics, thereby a magnetic field measurement signal is obtained from the feedback signal.
On the other hand, for a SQUID element itself, the SQUID element has various fundamental operating characteristics such as voltage-current (V-I) characteristics, the above mentioned V-.PHI. characteristics, or the like, and accordingly is verified or evaluated using these characteristics.
In addition, if a SQUID is formed with a high-temperature superconductor such as an oxide superconducting compound, the SQUID is allowed to operate at a relatively high temperature -196.degree. C. of liquid nitrogen (N), and accordingly to be handled easily. Accordingly, the system handling this high-temperature type of SQUID is remarkably simplified in contrast with a system using liquid helium (He) at a very low operating temperature of -269.degree. C.
Therefore, the high-temperature type of SQUID is expected to be developed and utilized in various applications, such as medical diagnosis, non-destructive testing, food inspection, geological survey, and the like, and many researchers will research and develop applications of the SQUID. However, in order to research and develop the applications in practice, it is necessary for these researchers to obtain a SQUID using a high-temperature superconductor, and so there is a hope for an easily available and experimental high-temperature superconducting SQUID.